1. Field of the Invention
The present invention relates to method of curing a mixture comprising an ionizing radiation curing resin, and a surface modifying technique.
2. Description of the Related Art
It is well known that solid surfaces having a low surface free energy exhibit high water repellence and oil repellence. Silicone oil or fluorine-type compounds are typical examples of substances having a low surface free energy, and are used as materials for modifying plastic, wood, fiber and metal surfaces (Japanese Patent Laid-Open 2000-191911, Japanese Patent Laid-Open Hei 5-117546 and many other references). Due to their characteristics, they are used to prevent snow building up on antenna or electrical wiring, to make it easier for ships to navigate by lowering resistance, and to prevent adhesion of water droplets or misting on car windows.
Solid surfaces having water repellence and oil repellence are used in various applications as described above, however as solid surfaces with water repellence and oil repellence also have reduced adhesion to adhesives and ink wettability, etc., decline, sufficient strength can no longer be obtained when they are stuck to other materials, and printing on them is difficult.
If the surface free energy of water-repellent surfaces and oil-repellent surfaces is partially increased, the part of the surface with the higher surface free energy can be modified to be hydrophilic or lipophilic, so adhesive properties and printing properties are improved, and the surfaces can be employed as planographic plate which make full use of differences in the tendency of ink to wet onto a solid surface. For example, by varying the surface free energy of an original plate wherein a photosensitive resin having hydrophobic properties is coated onto a hydrophilic surface comprising an aluminium plate, planographic plate can be performed using a planographic plate obtained by forming an image part with ink and a non-image part without ink. When the photosensitive resin is a negative photosensitive resin, the region irradiated by ionizing radiation becomes the image part, and when a positive photosensitive resin is used, the region irradiated by ionizing radiation becomes the non-image part. The surface modification of this image part and non-image part may be realized by developing and removing the photosensitive resin.
In Japanese Patent Laid-Open Hei 10-114888, an example is given where, by irradiating a super-water repellent surface having a contact angle with water of 150xc2x0 or more with ionizing radiation, a hydrophilic surface is obtained having a contact angle with water of 70xc2x0 or less which can be used as a printing plate. In Japanese Patent Laid-Open 2000-87016, by irradiating a super-water repellent surface having a contact angle with water of 150xc2x0 or more, a material is obtained having a contact angle with water of 10xc2x0 or less.
However, the above planographic plate and materials which were changed from super-water repellent to hydrophilic surfaces are all solids, and as they have little miscibility in solvents, it was difficult to apply them as coating agents.
It is therefore an object of this invention to make it possible to obtain curing surfaces with different surface free energies, and to provide a method of curing an ionizing radiation curing resin composition which can be used as a coating material. It is a further object of this invention to provide a surface modification method which permits surfaces of different surface free energy to be obtained.
According to first aspect of the invention, which resolves the aforesaid problems, is a method of curing a mixture comprising an ionizing radiation curing resin, characterized in that at least part of a mixture comprising 0.01-10 weight parts of a compound (b) having a surface free energy not exceeding 25 mN/m relative to 100 weight parts of an ionizing radiation curing resin composition (a) having a surface free energy of at least 30 mN/m, is cured by irradiating it with ionizing radiation in contact with a medium having a surface free energy higher than that of the ionizing radiation curing resin composition (a).
According to second aspect of the invention is the above method of curing a mixture comprising an ionizing radiation curing resin composition, wherein the medium having a surface free energy higher than that of the ionizing radiation resin curing composition (a), is water.
According to third aspect of the invention is a method of modifying a surface, comprising a step wherein a mixture comprising 0.01-10 weight parts of a compound (b) having a surface free energy not exceeding 25 mN/m relative to 100 weight parts of an ionizing radiation curing resin composition (a) having a surface free energy of at least 30 mN/m is coated on the surface of a substrate, and a step wherein the mixture is partially cured by irradiating it with ionizing radiation in contact with a medium having a surface free energy lower than that of the compound (b), and the remaining, uncured part is cured by irradiating it with ionizing radiation in contact with a medium having a surface free energy higher than that of the ionizing radiation curing resin composition (a).
According to fourth aspect of the invention is a method of modifying a surface, comprising a step wherein a mixture comprising 0.01-10 weight parts of a compound (b) having a surface free energy not exceeding 25 mN/m relative to 100 weight parts of an ionizing radiation curing resin composition (a) having a surface free energy of at least 30 mN/m is coated on the surface of a substrate, and a step wherein the mixture is partially cured by irradiating it with ionizing radiation in contact with a medium having a surface free energy higher than that of the ionizing radiation curing resin composition (a), and the remaining, uncured part is cured by irradiating it with ionizing radiation in contact with a medium having a surface free energy lower than that of the compound (b).
According to fifth aspect of the invention is the above method of modifying a surface, wherein the surface free energy after curing the part that was cured in contact with a medium having a surface free energy higher than that of the ionizing radiation curing resin composition (a), is higher by at least 5 mN/m than the part that was cured by irradiating it with ionizing radiation in contact with a medium having a surface free energy lower than that of the compound (b).
According to sixth aspect of the invention, is a method of modifying a surface comprising a step wherein a mixture comprising 0.01-10 weight parts of a compound (b) having a surface free energy not exceeding 25 mN/m relative to 100 weight parts of an ionizing radiation curing resin composition (a) having a surface free energy of at least 30 mN/m is coated on the surface of a substrate, and a step wherein a medium having a surface free energy higher than that of the ionizing radiation curing resin composition (a) is made to adhere partially to the mixture on the surface of the substrate, and the mixture is cured by irradiating it with ionizing radiation in contact with a medium having a surface free energy lower than that of the compound (b).
According to seventh aspect of the invention, is the above method of modifying a surface, wherein the surface free energy after curing the part wherein a medium having a surface free energy higher than that of the ionizing radiation curing resin composition (a) was made to adhere, is higher by at least 5 mN/m than the part wherein the medium was not made to adhere.
Herein, the value of the surface free energy in the specification is the value at temperature 20xc2x0 C., relative humidity 50 percent, and measured by the method described below.
There are various methods for measuring the surface free energy of a liquid, but in this specification, the values measured by the Wilhelmy method at a temperature of 20xc2x0 C. and a relative humidity of 50 percent were used as the surface free energy. The measuring principle of the Wilhelmy method is as follows. As shown in FIG. 1, a plate 2 is suspended from one arm of a balance 1, one end is immersed in a liquid 3 to be measured, a suitable load 4 is attached to the other end and the balance 1 is adjusted. In addition to gravity and buoyancy, the plate 2 is also subject to a force in the downward direction from the measurement liquid 3, so the following equation (1) may be written for the equilibrium state:
Load 4=Weight of plate 2xe2x88x92Buoyancy of plate 2+Force from liquid surfacexe2x80x83xe2x80x83(1) 
As the force due to the liquid surface is equivalent to the surface tension (=surface free energy of the liquid), the surface free energy of the liquid can be obtained by measuring the force due to the liquid surface. The material of the plate 2 may be platinum or glass, etc., and as the surface free energy does not change, it may be a material which is not corroded by the measurement liquid 3. In the measurements according to the specification, platinum is used as the material of the plate 2.
The surface free energy of the solid cannot be measured directly, but the surface free energy can be found using various types of liquid known in the art. A liquid drop on a solid surface has the cross-sectional shape shown in FIG. 2. In the figure, a contact angle 9 is an angle subtended by a tangent 8 to the liquid 6 and the side of the surface of the solid 5 in contact with the liquid 6, at an intersection point 7 between the surface of the liquid 6 on the surface of the solid 5, and the surface of the solid 5. Let this angle be xcex8. The following equation (2) (Young""s equation) is then satisfied due to the equilibrium conditions at the intersection point 7:
"Ugr"S="Ugr"SL+"Ugr"LCOSxcex8xe2x80x83xe2x80x83(2) 
Here, "Ugr"S is the surface free energy of the solid, "Ugr"L is the surface free energy of the liquid, and "Ugr"SL is the surface free energy of the solid/liquid interface.
The surface free energy "Ugr" is represented by the sum of the dispersion force component "Ugr"a, polar force component "Ugr"b and hydrogen bond force component "Ugr"c, so the following assumption (3) can be made regarding the surface free energy of the solid/liquid interface "Ugr"SL:
"Ugr"SL="Ugr"S+"Ugr"Lxe2x88x922("Ugr"Sa"Ugr"La)xc2xdxe2x88x922("Ugr"Sb"Ugr"L b)xc2xdxe2x88x922("Ugr"Sc"Ugr"L c)xe2x80x83xe2x80x833) 
Therefore, if the value ? of the contact angle 9 is found for three or more liquids having different surface free energies under the conditions of temperature 20xc2x0 C. and relative humidity 50%, "Ugr"Sa, "Ugr"Sb, "Ugr"SC can be deduced by solving the equation with three unknowns derived from equation (2) and equation (3), and the surface free energy of the solid "Ugr"S can be found as their sum.